Audio production can involve the use of many components, including microphones, wireless audio transmitters, wireless audio receivers, recorders, and/or mixers for capturing, recording, and presenting the sound of productions, such as television programs, newscasts, movies, live events, and other types of productions. The microphones typically capture the sound of the production, which is wirelessly transmitted from the microphones and/or the wireless audio transmitters to the wireless audio receivers. The wireless audio receivers can be connected to a recorder and/or a mixer for recording and/or mixing the sound by a crew member, such as a production sound mixer. Electronic devices, such as computers and smartphones, may be connected to the recorder and/or mixer to allow the crew member to monitor audio levels and timecodes.
Wireless audio transmitters, wireless audio receivers, wireless microphones, and other portable wireless communication devices include antennas for transmitting radio frequency (RF) signals which contain digital or analog signals, such as modulated audio signals, data signals, and/or control signals. Users of portable wireless communication devices include stage performers, singers, actors, news reporters, and the like. One common type of portable wireless communication device is a wireless bodypack transmitter, which is typically secured on the body of a user with belt clips, straps, tape, etc. Another common type of portable wireless communication device is a wireless handheld microphone that is held by the user and includes an integrated transmitter and antenna.
Wireless transmitters may utilize frequency diversity to simultaneously transmit on one antenna two RF signals of two separate frequencies in a combined RF signal, where the two RF signals both include the same audio signal. This can help ensure that a wireless receiver that receives the combined RF signal will be able to process the audio signal in an uninterrupted manner by using one or both of the underlying RF signals. For example, in the case of interference on one of the RF signals on one frequency, the wireless receiver may utilize the other RF signal on the other frequency. Therefore, a wireless transmitter utilizing frequency diversity can modulate an audio signal into two incoherent RF signals of different frequencies, then combine the two RF signals together into a combined RF signal for transmission on a single antenna.
When combining two incoherent RF signals, it is desirable to isolate the two RF signals from one another as much as possible to reduce intermodulation as this is more spectrally efficient, i.e., a more optimal use of spectrum. As such, larger intermodulation products can lead to reduction of available spectrum for other usage as well as interference and/or reduced audio clarity for other wireless transmitters sharing the same spectrum. Using a Wilkinson combiner, for example, to combine the two RF signals leverages resistance between the RF signals to increase isolation and reduce intermodulation. Wilkinson combiners are typically implemented in single or multiple stages, depending on the needs of a particular application. However, while a Wilkinson combiner can provide high isolation, it also has a limited bandwidth that may not be suitable for use in certain applications and environments. In particular, the limited bandwidth of a typical Wilkinson combiner may hamper a wireless transmitter from being able to take advantage of available spectrum and thus be spectrally inefficient. In addition, a filter on the output of a multiple stage Wilkinson combiner may influence the ability to achieve a desired transmitter bandwidth while maintaining sufficient isolation between the RF signals and balanced impedance between stages of the combiner.
Accordingly, there is an opportunity for a combiner system that addresses these concerns. More particularly, there is an opportunity for a wideband tunable combiner system including a tunable capacitance network for improving isolation and reducing intermodulation in a dual frequency diversity transmitter.